This invention relates to the field of ceramics and particularly to ceramics having ionic impurities in a glassy phase.
Ceramic materials such as silicon nitride are frequently used for parts which require good high temperature strength, or low dielectric losses, or a combination of both high temperature strength and low dielectric losses. There is a continuing need to improve the properties of these ceramics so that they can be used more effectively at higher stresses and at higher temperatures.
Ceramics such as silicon nitride are generally prepared by mixing the ceramic powder with a densification aid and then hot pressing the mixture to form the densified ceramic. During hot pressing, the densification aid melts to form a liquid which, upon cooling, forms a glassy intergranular phase. Unfortunately, this glassy phase becomes viscous at high temperatures and causes a reduction in mechanical properties. The temperature at which degradation begins depends upon the composition of the glassy phase. The extent of degradation depends upon the volume fraction of the glassy phase. Within a given system, compositions containing a smaller volume fraction of glassy phase have a higher strength at high temperatures. However, compositions containing little or no glassy phase arae difficult or impossible to densify.
The glassy phase contains ionic impurities which are difficult to remove and ionic additives which promote the densification of the powder during fabrication. Unfortunately, these ions lower the melting point of the intergranular glassy phase, thus reducing its high temperature strength, and they reduce its dielectric properties particularly at high temperatures.